There is an illuminance sensor which is disclosed in JP-A-2004-506569 corresponding to U.S. Pat. No. 6,800,959. The constitution of the sensor is as follows. Outside light is transmitted through a windshield of a vehicle and a second light guide, and is received by a first receiver. A light emitted from a transmitter is entered to the windshield through a first light guide, and the light is reflected at a surface of the windshield opposite to the first light guide. And then, the reflected light is transmitted once again through the windshield and the first light guide, and is received by a second receiver.
Transmittance of a windshield and a light guide may be decreased due to aging with time. In such a case, it becomes difficult to detect the brightness of outside the vehicle correctly, because a level of a detected signal from the first receiver changes even if the brightness of outside the vehicle is not changed.
The above-mentioned illuminance sensor calculates transmittance of the windshield and the first light guide based on the detected signal from the second receiver. The brightness of outside is detected correctly by correcting the detected signal from the first receiver based on the calculated transmittance of the windshield and the first light guide.
In addition, in the above-mentioned illuminance sensor, the transmitter and the second receiver are used as a raindrop sensor for detecting the amount of raindrop attached to the windshield. When the raindrop is not attached to the windshield, the light from the transmitter is totally reflected at a surface of the windshield. On the other hand, when the raindrop is attached to the windshield, a part of the light from the transmitter is outputted from the windshield through the raindrop. Due to this, the amount of inputted light is decreased compared to the case that the raindrop is not attached to the windshield. Therefore, it is possible to calculate the amount of raindrop based on the detected signal from the second receiver.
In the above-described illuminance sensor, a region in which the outside light is transmitted to send the light to the first receiver is different from a region in which light emitted from the transmitter is transmitted to send the light to a third receiver. That is, the outside light which enters the first receiver and the light from the transmitter which enters the third receiver are passing through different portions, respectively. Therefore, it is difficult to detect the brightness of outside correctly because the transmittances of the two different portions are different from each other, even if the detected signal from the first receiver is corrected.
In a case where a raindrop sensor is assembled into an illuminance sensor integrally, the raindrop sensor should be positioned in the region in which a wiping operation of a wiper is performed because of their function, and in the region which does not block the driver's vision. In order to satisfy these conditions, the raindrop sensor is arranged at the back of a rear-view mirror. This position may correspond to a gradation part of the windshield which is tinted for anti-dazzling. In such a case, it is difficult to detect the brightness of outside correctly, even if the detected signal from the first receiver is corrected.